Wind information can be reported by various sources (ATIS, METAR, TAF, spoken on the radio, etc). I was taught that officially some of these sources are relative to magnetic north and others to true north (although practically this isn't always followed). Which sources are supposed to use which reference system?
The general rule is:
If you read it, it's true. If you hear it, it's magnetic.
All charts and textual sources (METAR, TAF, winds aloft, surface analysis charts, etc) use true north as the reference.
ATIS/AWOS/ASOS broadcasts, or any information a controller gives you over the radio, is magnetic.
Wind direction broadcast over FAA radios is in reference to magnetic north.
AIM Section 7-1-11 (page 7-1-26 in the 5/26/16 edition)
One exception to the "if you hear it" rule is that a FSS briefer will read you the winds referenced to true north, since they're just reading you the charts/textual information.
(This is at least true in the United States... other countries may vary in some instances)
In the United States, the principal guide to weather products is AC 00-45G "Aviation Weather Services". As of this writing it appears Change 1 is current, published July 2010. Citations given here are to that revision. The Airman's information manual has information on AWOS and ASOS.
Here are some examples:
- METAR: True 18.104.22.168
- AWOS & ASOS: True AIM 7-1-11 Figure 7-1-5
- TAF: True 22.214.171.124
- WA (Winds Aloft): True 7.4.2
- UA (Pilot Reports): Magnetic 126.96.36.199 (I'd suppose you get whatever the pilot reports.)
- Surface Analysis Chart: True 188.8.131.52.5
When calling flight service, the briefer will have available the same information products that you can reach on aviationweather.gov. I expect they will know the direction reference for the information they are giving you.
The way I remember it is that if it relates to an aerodrome its magnetic, because runway numbers are in magnetic and the pilot needs to know how the wind relates to the runway in use so the amount of headwind / crosswind can be assessed.
If it relates to en-route then its in true, because flight planning is done in true (with conversions to the correct magnetic heading to set on the compass at each step).